1. Field of the Invention
The field of the invention is data processing, or, more specifically, cables and methods of manufacturing cables for high speed data communications.
2. Description of Related Art
High speed data communications over shielded cables are an important component to large high-end servers and digital communications systems. While optical cables provide long distance drive capability, copper cables are typically preferred in environments that require a shorter distance cable due to a significant cost savings opportunity. A typical copper cable used in environments requiring a shorter distance cable, is a twinaxial cable. A twinaxial cable is a coaxial cable that includes two insulated, inner conductors and a shield wrapped around the insulated inner conductors. Twinaxial cables are used for half-duplex, balanced transmission, high-speed data communications. In current art however, twinaxial cables used in data communications environments are limited in performance due to a bandstop effect.
For further explanation of typical twinaxial cables, therefore, FIG. 1 sets forth a perspective view of a typical twinaxial cable (100). The exemplary typical twinaxial cable (100) of FIG. 1 includes two conductors (106, 108) and two dielectrics (110, 112) surrounding the conductors. The conductors (106, 108) and the dielectrics (110, 112) are generally parallel to each other and a longitudinal axis (105).
The typical twinaxial cable (100) of FIG. 1 also includes a shield (114). The shield, when wrapped around the conductors of a cable, acts as a Faraday cage to reduce electrical noise from affecting signals transmitted on the cable and to reduce electromagnetic radiation from the cable that may interfere with other electrical devices. The shield also minimizes capacitively coupled noise from other electrical sources, such as nearby cables carrying electrical signals. The shield (114) is wrapped around the conductors (106, 108). The shield (114) includes wraps (101-103) along and about the longitudinal axis (105), each wrap overlapping the previous wrap. A wrap is a 360 degree turn of the shield around the longitudinal axis (105). The typical twinaxial cable of FIG. 1 includes three wraps (101-103), but readers of skill in the art will recognize that the shield may be wrapped around the inner conductors and the dielectric layers any number of times in dependence upon the length of the cable. Wrap (101) is shaded for purposes of explanation. Each wrap (101-103) overlaps the previous wrap. That is, wrap (101) is overlapped by wrap (102) and wrap (102) is overlapped by wrap (103). The overlap (104) created by the overlapped wraps is continuous along and about the longitudinal axis (105) of the cable (100).
The wraps (101-103) of the shield (114) create an overlap (104) of the shield that forms an electromagnetic bandgap structure (‘EBG structure’) that acts as the bandstop filter. An EBG structure is a periodic structure in which propagation of electromagnetic waves is not allowed within a stopband. A stopband is a range of frequencies in which a cable attenuates a signal. In the cable of FIG. 1, when the conductors (106, 108) carry current from a source to a load, part of the current is returned on the shield (114). Due to skin effect, the current in the conductors to the load displaces on the outer surface of the conductor, and the current return path attempts to run parallel to, but in the opposite direction of, the current to the load. As such, the current on the shield (114) encounters the overlap (104) of the shield (104) periodically and a discontinuity exists in the current return path due to the overlap. The discontinuity in the current return path at the overlap (104) created by the wraps (101-103) acts as a bandstop filter that attenuates signals at frequencies in a stopband.
For further explanation, FIG. 2 sets forth a cross-sectional view of a prior art data communications cable (100), similar to a twinaxial cable. The cable (100) in the example of FIG. 2 only depicts a single conductor (108) for clarity of explanation only, but readers of skill in the art will immediately recognize that two or more conductors may be present in such a cable. The typical twinaxial cable of FIG. 2 includes a shield (114), surrounding a conductor (108) insulated with a dielectric layer (109). Current (204) in the example of FIG. 2 is flowing in the conductor (108) on the ‘skin’ or outer-edge of the conductor (108) while the return current (208) flows in the opposite direction along the conductive shield (114). The shield (114) includes two conductive layers (206) separated by a dielectric layer. The current return path, attempting to run parallel to the current (204) in the conductor, travels from right to left as depicted by the repeating arrows, and must ‘jump’ the dielectric layers (208) of the shield (114) to flow closer to the ‘skin’ of the conductor. Such a ‘jump’ creates a capacitance, two charged plates separated by a dielectric medium, which periodically repeats along the length of the cable. The cable also has some internal inductance due to various factors, including the repetitive wrapping of a conductive shield around the conductor (108).
For further explanation, therefore, FIG. 3 sets forth a graph of the insertion loss of a typical twinaxial cable. Insertion loss is the signal loss in a cable that results from inserting the cable between a source and a load. The insertion loss depicted in the graph of FIG. 3 is the insertion loss of a typical twinaxial cable, such as the twinaxial cable described above with respect to FIG. 1. In the graph of FIG. 3, the signal (119) is attenuated (118) within a stopband (120) of frequencies (116) ranging from seven to nine gigahertz (‘GHz’). The stopband (120) has a center frequency (121) that varies in dependence upon the composition of the shield, the width of the shield, and the rate that the shield is wrapped around the conductors and dielectrics. The center frequency (121) of FIG. 3 is 8 GHz.
The attenuation (118) of the signal (119) in FIG. 3 peaks at approximately −60 decibels (‘dB’) for signals with frequencies (116) in the range of approximately 8 GHz. The magnitude of the attenuation (118) of the signal (119) is dependent upon the length of the cable. The effect of the EBG structure, the attenuation of a signal, increases as the length of the EBG structure increases. A longer cable having a wrapped shield has a longer EBG structure and, therefore, a greater attenuation on a signal than a shorter cable having a shield wrapped at the same rate. That is, the longer the cable, the greater the attenuation of the signal. In addition to signal attenuation, the bandstop effect also increases other parasitic effects in the cable, such as jitter and the like.
Typical twinaxial cables for high speed data communications, therefore, have certain drawbacks. Typical twinaxial cables have a bandstop filter created by overlapped wraps of a shield that attenuates signals at frequencies in a stopband. The attenuation of the signal increases as the length of the cable increases. The attenuation limits data communications at frequencies in the stopband.